The invention relates to a process for the hydrogenation of a polymer composed of diene monomer units and nitrile group containing monomer units, as well as to hydrogenated polymer obtained.
Of polymers that contain conjugated diene monomer units and a nitrile group containing monomer unit, such as for instance nitrile butadiene rubber (NBR), the double bonds present in the chain can be hydrogenated, so that for instance the polymers become less sensitive to oxidative degradation, in particular oxidative degradation at elevated temperature. Such hydrogenated polymers are applied, for instance, under the bonnet in cars.
Commercial processes for the preparation of such hydrogenated polymers are very cumbersome and expensive. The polymer containing diene monomer units and nitrile group containing monomer units is for instance dissolved in a suitable solvent and then hydrogenated with hydrogen gas at a high pressure and a high temperature in the presence of a noble metal catalyst. The catalyst and the solvent must subsequently be removed. In addition, when high degrees of hydrogenation are achieved side reactions take place that result in undesirable branching and even gelation.
U.S. Pat. No. 4,452,950 discloses the hydrogenation of polymers containing conjugated diene monomer units and a nitrile group containing monomer unit, in the form of an aqueous dispersion, with the aid of hydrazine. The polymers in themselves are usually already present as an aqueous dispersion in the form of a latex after the polymerization, or can be given the form of an aqueous dispersion. This in itself yields a simplified process for the hydrogenation, but the process also has the drawback that crosslinking takes place in the hydrogenated polymer already during the hydrogenation reaction, but especially also after the hydrogenation reaction, when the hydrogenated polymer is coagulated, separated and dried and also afterwards, during storage. This causes for instance the rheological properties of the hydrogenated polymer to change in an uncontrolled manner, which has an adverse effect on further compounding and processing into moulded articles. It is even possible for the hydrogenated polymer to be entirely unsuitable for further use, because the crosslinking reaction has proceeded to such an extent that the hydrogenated polymer contains gel particles or is even entirely crosslinked.
It is the aim of the invention to provide a process that does not have the above-mentioned drawbacks or has them only to a strongly reduced extent.
Surprisingly, this aim is achieved in that crosslinks formed due to the hydrogenation are broken by adding, before, during or after the hydrogenation, a compound that satisfies formula I or formula II 
where
R1 is a hydrogen atom, an alkyl or cycloalkyl group with 1-30 carbon atoms, or an aromatic group with 6-30 carbon atoms, and
R2 is an alkyl or cycloalkyl group with 1-30 carbon atoms, or an aromatic group with 6-30 carbon atoms, X is chosen from the group comprising xe2x80x94R3, xe2x80x94OR4, xe2x80x94SR4, xe2x80x94NR5R6, where R3, R4 and R5 are a hydrogen atom, an alkyl or cycloalkyl group with 1-30 carbon atoms or an aromatic group with 6-30 carbon atoms and R6 is an alkyl or cycloalkyl group with 1-30 carbon atoms or an aromatic group with 6-30 carbon atoms
Y is chosen from the group comprising xe2x80x94R7, xe2x80x94OR8, xe2x80x94SR8, xe2x80x94NR9R10 and xe2x80x94Nxe2x95x90CR11R12 where R7, R8, R9, R10, R11 and R12 are a hydrogen atom, an alkyl or cycloalkyl group with 1-30 carbon atoms or an aromatic group with 6-30 carbon atoms, and it being possible for R3-R12 to also contain one or more heteroatoms from the groups 13, 14, 15, 16 or 17 of the Periodic System of the Elements.
The Periodic System of the Elements is understood to be Periodic System according to the IUPAC nomenclature, shown on the inside of the cover of the Handbook of Chemistry and Physics, 67th edition, 1986-1987.
The addition of such a compound causes strong or complete suppression of the crosslinking reaction. Moreover, addition of the compounds according to the invention has the advantage that no ozonolysis needs to be carried out in order to break crosslinked polymer chains.
It is true that U.S. Pat. No. 5,442,009 discloses ozone treatment of the hydrogenated and crosslinked polymer, so that chains of the hydrogenated polymer are broken and the effect of the crosslinking reaction is wholly or partly eliminated. After this treatment, however, a second crosslinking reaction occurs. According to U.S. Pat. No. 5,039,737 this second crosslinking reaction can be suppressed by treating the hydrogenated and ozone-treated hydrogenated polymer with hydroxylamine, but this is in itself already cumbersome and the combined treatments to which the hydrogenated polymer is to be subjected thus again become complex and expensive, so that no good alternative is obtained to the existing commercial processes for the hydrogenation of polymers containing conjugated diene monomer units and a nitrile group containing monomer unit. Furthermore, the ozone treatment is cumbersome. The second crosslinking reaction is a different type of reaction from the first crosslinking reaction. There is no indication for the use of hydroxylamine also for suppression of the first crosslinking reaction, the more so since hydroxylamine is used only to react with unstable terminal aldehyde groups to form stable oxime compounds. Moreover, the addition of hydroxylamine in a process for the hydrogenation of said polymers by the action of a copper catalyst as in U.S. Pat. No. 5,039,737, but without an ozonolysis being carried out, does not result in the crosslinks formed being broken and a gel-free, hydrogenated polymer being obtained.
Preferably, compounds are used in which R, is a hydrogen atom.
Examples of compounds that satisfy formula I are primary and secondary amines, hydrbxylamine, derivatives of hydroxylamine and substituted hydrazines, dithiocarbamylsulphenamide compounds, thiuram compounds and dithiocarbamate compounds. Specific examples are methylamine, ethylene diamine, dodecylamine, ethanolamine, cyclohexyldiamine, o-phenylene diamine, 3,4-toluene diamine, 1,8-naphthalene diamine, aniline, methylhydrazine, phenylhydrazine, o-aminophenol, o-aminobenzoic acid, hydroxylamine, N-isopropyl hydroxylamine, O-methylhydroxylamine, O-t-butylhydroxylamine and the sulphur compounds tetramethylthiuram disulphide, N-oxydiethylene dithiocarbamyl-Nxe2x80x2-oxydiethylene-sulphenamide. Preferably, use is made of compounds that satisfy formula I where R3 is an aromatic group with 6-15 carbon atoms, R4 is a hydrogen atom, or an alkyl group with 1-5 carbon atoms, R5 is a hydrogen atom, an alkyl group with 1-6 carbon atoms or an aromatic group with 6-10 carbon atoms and R6 is an alkyl group with 1-6 carbon atoms or an aromatic group with 6-10 carbon atoms, it being possible for R3-R6 to also contain one or more heteroatoms from the groups 13, 14, 15, 16 or 17 of the Periodic System of the Elements. Most preferably, hydroxylamine or orthoaromatic diamines are used.
Examples of compounds that satisfy formula II are imines, azines, hydrazones, semicarbazones, oximes and benzothiazoles. Specific examples are N-phenylbutyl imine, N-isopropylbenzaldehyde imine, acetone azine, benzaldehyde azine, cyclohexanone azine, benzaldehyde hydrazone, benzophenone hydrazone, benzaldehyde oxime, p-nitrobenzaldehyde oxime, o-, p-, and m-chlorobenzaldehydeoxime, cyclohexanone oxime, acetonoxime, 2-mercaptobenzothiazole, N-cyclohexyl-2-benzothiazole sulphenamide methyl ethyl ketone oxime, benzophenone oxime.
Preferably, compounds are used that satisfy formula II, where R7 is an aromatic group that contains 6-10 carbon atoms, R8 is a hydrogen atom or an alkyl group with 1-6 carbon atoms, R9-R11 are a hydrogen atom, an alkyl group with 1-6 carbon atoms, or an aromatic group with 6-10 carbon atoms, and R12 an alkyl group with 1-6 carbon atoms or an aromatic group with 6-10 carbon atoms, it being possible for R7-R12 to also contain one or more heteroatoms from the groups 13, 14, 15, 16 or 17 of the Periodic System of the Elements. More preferably, use is made of compounds that satisfy formula II where Y is an xe2x80x94OHxe2x80x94 group. Most preferably, use is made of a compound that satisfies formula II where Y is an OH group, R1 a hydrogen atom and R2 an alkyl or cycloalkyl group with 1-15 carbon atoms or an aryl group with 6-10 carbon atoms. Examples of these compounds are benzaldehyde oxime, propionaldehyde oxime and dodecylaldehyde oxime.
Compounds that satisfy the description of formula I are preferably added after the hydrogenation reaction, more preferably before or during separation of the hydrogenated polymer, most preferably to the hydrogenated polymer during a post-treatment. Surprisingly, this suppresses the crosslinking reaction in the hydrogenated polymer even further. In particular, stronger suppression of the crosslinking during storage takes place. This is important for instance if a hydrogenated polymer with a high molecular weight is used. The occurrence of a crosslinking reaction has a strong effect on the rheological properties of such a polymer.
Hydroxylamine is preferably added to the hydrogenated polymer in the form of an aqueous solution. Most preferably, the hydrogenated polymer is contacted with a 1-20 wt. % solution of hydroxylamine in water.
More preferably, a combination of an oxime, of the general formula II, where Y is an OH group, R1 a hydrogen atom and R2 an alkyl or cycloalkyl group with 1-15 carbon atoms or an aryl group with 6-10 carbon atoms, and hydroxylamine is added. The oxime is then preferably added before or during the hydrogenation of the polymer, while the hydroxylamine is preferably added during post-treatment of the hydrogenated polymer crumb.
Even more preferably, a compound according to formula I or II is present during kneading of hydrogenated polymer at elevated temperature. The kneading temperature preferably lies between 50 and 300xc2x0 C., most preferably between 100 en 200xc2x0 C.
Most preferably, the hydrogenated polymer is kneaded in the presence of a compound according to formula I, with also a carbonyl-group containing compound (for instance an aldehyde or ketone) being present. Most preferably, a ketone is used. Examples of suitable ketones are acetophenone, benzophenone and 2-dodecylketone. This further accelerates the degradation of crosslinks by the action of the compounds according to the invention, so that the kneading time and/or the kneading temperature can be reduced. Special preference is given to o-aromatic diamines as the compounds according to formula I in combination with a ketone. Preferably, o-phenylene diamine or 3,4-toluene diamine are used.
The polymer that is used in the process according to the invention can be obtained by the polymerization of diene monomers, nitrile group containing monomer units and optionally other monomers, such as for instance acrylates or methacrylates. As the conjugated diene monomers use can for instance be made of 1,3-butadiene, 2,3-dimethylbutadiene, isoprene and 1,3-pentadiene. As the nitrile group containing monomer units use can for instance be made of acrylonitrile and methacrylonitrile. Preferably, 1,3-butadiene and acrylonitrile are used.
The polymer preferably contains 5-95 wt. % of one or more conjugated diene monomers and 95-5 wt. % of a nitrile group containing monomer unit. More preferably, the polymer contains 40-90 wt. % of butadiene monomer and 60-10 wt. % of acrylonitrile.
The polymer can be prepared using various processes which in themselves are known from the state of the art: examples of suitable processes are emulsion polymerization, solution polymerization or bulk polymerization.
Preferably, the polymer has been prepared by means of an emulsion polymerization in water, so that the polymer becomes available as an aqueous dispersion, even more preferably as a latex. U.S. Pat. No. 5,442,009 gives clear instructions for the preparation of a polymer, the conditions to be used, the initiators to be used, etc. If the polymer does not become available in the form of a latex, then the polymer is preferably introduced into an aqueous dispersion.
The polymer is preferably subjected to the hydrogenation in the form of the aqueous dispersion, use being made preferably of a diimide, or a compound forming a diimide and optionally a catalyst.
Preferably, hydrazine is used as the compound forming a diimide, with the diimide being formed in the presence of an oxidizing agent. Examples of suitable oxidizing agents are air, oxygen, ozone, peroxides, hydroperoxides, iodine, iodates, hypochlorite and similar compounds. Particularly suitable oxidizing agents are chosen from the group comprising peroxides and hydroperoxides. Most preferably, use is made of hydrogen peroxide.
The ratio of the molar amount of hydrazine relative to peroxide or hydroperoxide is preferably between 0.9 and 1.2.
The oxidizing compound is for instance present in a molar ratio of 0.1:1 to 100:1 relative to the carbonxe2x80x94carbon double bonds. Preferably, this ratio is between 0.8:1 and 5:1, most preferably between 0.9:1 and 1.5:1.
Preferably, a catalyst is used which contains an element from group 13 of the Periodic System of the Elements. Very good results are obtained if boron is chosen as the element from group 13. Even more preferably, the catalyst is chosen from the group comprising borates, peroxyborates and boric acid (H3BO3). Most preferably, boric acid is used.
The amount of hydrogenation is the percentage of carbonxe2x80x94carbon double bonds that is saturated after the hydrogenation reaction compared with the initial amount of double bonds. The process of the present invention provides polymers that for instance have an amount of hydrogenation higher than 60%. Preferably, the amount of hydrogenation is higher than 80%. The process is eminently suitable for the preparation of polymers with an amount of hydrogenation higher than 90%, because the present process provides hydrogenated polymers that can advantageously be prepared and have a low gel content.
The hydrogenation reaction temperature is between 0xc2x0 and 250xc2x0 C. Preferably, the temperature is between 200 and 150xc2x0 C. Special preference is given to a reaction temperature of 300 to 80xc2x0 C.
During the hydrogenation in a latex a small amount of solvent may be present for the unsaturated polymer. In that case the amount of solvent may for instance vary between 0 and 20 wt. % (solvent relative to polymer).
After the hydrogenation reaction the hydrogenated polymer can be separated from the solvent or from the latex. Examples of suitable separation processes are precipitation and steam stripping. Particularly precipitation is a suitable purification method, for which standard processes can be used, for instance the addition of an acid or salt to the aqueous dispersion of the polymer, or administration of an alcohol. Preferably, use is made of an alcohol or a magnesium sulphate solution in water for coagulation of the hydrogenated polymer latex.
After separation of the hydrogenated polymer a mixture is obtained of polymer crumb and water, which may contain all kinds of compounds, such as hydrazine, peroxide and soap residues, etc. Before or during the separation compounds according to the present invention can be added, as well as all kinds of additives, for instance stabilizers, dyestuffs, plasticizers, etc.
Preferably, before or after hydrogenation of the polymer stabilizers are added that are chosen from the group comprising free radical stabilizers, for instance phenolic antioxidants (such as for instance 2,6-di-tert.butyl-p-cresol or 2,2xe2x80x2-methylene bis(4-methyl-6-tert.butyl phenol)), amine antioxidants (such as for instance p-phenylene diamine derivatives, N-isopropyl-Nxe2x80x3-phenyl-p-phenylene diamine), dihydroquinoline derivatives (such as for instance 2,2,4-trimethyl-1,2-dihydroquinoline), benzimidazole derivatives (such as for instance 2-mercaptobenzimidazole) and naphthylamine derivatives (such as for instance phenyl-alpha-naphthylamine). Addition of such stabilizers yields the advantage that the stability of the hydrogenated polymer is improved even further, so undesirable gel formation will no longer occur during storage and further processing of the hydrogenated polymer, either.
After the separation the polymer can be worked up, for instance by filtering the hydrogenated polymer, drying in it in an oven, in a fluid-bed drier, etc., with drying optionally taking place at reduced pressure.
If post-treatment of the hydrogenated polymer crumb is applied, a short and/or incomplete working up procedure may be used. The hydrogenated polymer can for instance be subjected to a short working up procedure when the post-treatment involves stirring of the polymer crumb in an aqueous solution of a compound according to the present invention. In such a case a simple filtration step suffices, so that a wet crumb is obtained that is suitable for further use.
If a kneading step is applied as post-treatment, it may, however, be desirable to use the hydrogenated polymer in virtually moisture-free condition, so that working up of the hydrogenated polymer preferably includes a drying step. Kneading can for instance take place in a Brabender or an extruder. This preferably involves the addition of a compound that satisfies formula I or II in an amount of 0.1-10 wt. % on the basis of the polymer, most preferably 0.5-2%.
The invention also relates to a polymer composition in solid form that contains hydrogenated polymer composed of diene monomer units and nitrile group containing monomer units, the hydrogenated polymer having an xcex1/xcex2 peak ratio  greater than 1, measured by means of 1H-NMR, the xcex1 peak having been measured between 2.05 and 2.2 ppm and the xcex2 peak between 1.8 and 2.05 ppm, a gel content of less than 15 wt. % and a molecular weight distribution (Mw/Mn, measured using gel permeation chromatography (GPC)), of the soluble part of the polymer composition of  less than 10.
Hydrogenated polymers with an xcex1/xcex2 peak ratio  greater than 1 have an improved thermal and oxidative stability.
It is true that a process involving the hydrogenation of NBR in solution, followed by solvent removal, as for instance disclosed in EP 174 551 A, also yields a polymer composition that contains an H-NBR with a low gel content. However, this involves a cumbersome hydrogenation process, the polymer composition may contain solvent residues and the process results in a different type of H-NBR, with for instance an NMR spectrum that deviates from the H-NBR of the polymer composition according to the invention. An H-NBR prepared according to EP 174-551 A for instance has an xcex1/xcex2 peak ratio  less than 1.
Preferably, the polymer composition according to the invention contains an H-NBR with a gel content of at most 10 wt. %, even more preferably at most 5 wt. %.
Preferably, the soluble part of the polymer composition has a weight average molecular weight (Mw), measured by GPC, higher than 100,000.
A further advantage of the polymer composition according to the invention is that the gel content still or at least still has a low gel content even after a very long storage time.
The polymer composition according to the invention may contain additives, such as for instance stabilizers, chosen from the group comprising free radical stabilizers, phenolic antioxidants, amine antioxidants, dihydroquinoline derivatives, benzimidazole derivatives and naphthylamine derivatives. The polymer composition may for instance also contain reinforcing fibres or a second polymer besides H-NBR.
The polymer composition according to the invention may for instance be injection moulded to form moulded articles. Examples of moulded articles, in which the polymer composition according to the invention shows up very well are automotive parts to be mounted in the engine compartment.
Below, the invention will be elucidated with reference to some examples, without however being limited to these examples.
The amount of hydrogenation of hydrogenated polymers was determined by means of 1H-NMR on a Brucker AC-200 Mhz. The determination of the amount of hydrogenation took place as described in Rubber Chemistry and Technology, 1990, Vol 63, p. 245.
Soxhlet extractions were performed with chloroform as the extraction agent and in a Whatman cellulose extraction thimble with an internal diameter of 19 mm and a length of 90 mm. The thimble was filled with about 0.5 grams of polymer, after which the extraction was carried out for 6 hours. The gel content was determined on the basis of the weight increase of the thimble after evaporation of the chloroform and stabilization of the thimble weight.
Experiment A
38.8 grams of hydrazine monohydrate, to which a mixture of 5 grams of boric acid, 40 grams of water and a few drops of silicon oil had been added, were added to 200 grams of latex Nysin(trademark) 33-3, with a solids content of 25%, a butadiene content of 67 wt. %, an acrylonitrile content of 33 wt. %, and a Mooney125xc2x0 C.,1+4 of 30. After the temperature had been adjusted to 40xc2x0 C., in 6 hours 89 grams of a 30% m/m (=on a mass basis) solution of hydrogen peroxide in water were added. The polymer was [ . . . ] in 800 grams of a 0.5% m/m solution of MgSO4. 7H2O. Immediately upon precipitation and upon vacuum drying at room temperature in about 16 hours the polymer was found to be insoluble in chloroform.